The invention relates generally to a system and method for operating a compression-ignition engine and, more specifically, for controlling emissions.
Compression-ignition engines, such as diesel engines, operate by directly injecting a fuel (e.g., diesel fuel) into compressed air in one or more piston-cylinder assemblies, such that the heat of the compressed air lights the fuel-air mixture. The direct fuel injection atomizes the fuel into droplets, which evaporate and mix with the compressed air in the combustion chambers of the piston-cylinder assemblies. The fuel efficiency, exhaust emissions, and other engine characteristics are directly affected by the compression ratio, the fuel-air ratio, injection timing, ambient conditions, and so forth. Exhaust emissions include pollutants such as carbon monoxide, oxides of nitrogen (NOx), particulate matter (PM), and smoke generated due to incomplete combustion of fuel within the combustion chamber.
Unfortunately, fuel efficiency, exhaust emissions, and other operational characteristics are less than ideal. In addition, conventional techniques to improve one operational characteristic often worsen one or more other operational characteristic. For example, attempts to decrease specific fuel consumption often cause increases in various exhaust emissions. Existing emissions control schemes generally take a conservative approach to ensure emissions compliance, thereby resulting in unnecessarily low fuel efficiency. For example, existing emissions control schemes often use static look-up tables based on previous operational data. Unfortunately, the actual operation of the engine may vary significantly from the static look-up tables, particularly after significant use and wear on the engine and also due to engine power production variation. As a result, the engine exhaust emissions may be at greater or lesser levels than expected by the static look-up tables. Again, the specific fuel consumption is also affected by the emissions control schemes.